1. Field of the Invention
The present invention relates to an observation apparatus such as a pair of binoculars, and more particularly to an observation apparatus which has a mechanism of moving a part of an observation optical system in an optical axis direction.
2. Description of the Related Art
An observation apparatus such as a pair of binoculars is an optical apparatus which is able to observe an optical image of an object to be observed that an objective optical system forms by enlarging the optical image with an eyepiece optical system. An optical element constituting a part of the objective optical system or the eyepiece optical system moves in an optical axis direction to change an observation magnification or perform a focusing for the object to be observed.
Japanese Patent Laid-Open No. 10-319325 discloses a pair of binoculars which moves a pair of left and right objective lenses integrally attached to an objective mount in an optical axis direction to perform a focusing. The objective mount slides on a convex portion formed at a base of the pair of binoculars. Further, each of two guide members fixed on the base engages with corresponding one of two guide holes, which are formed in the objective mount extending in the optical axis direction and away from each other in the optical axis direction, in a direction orthogonal to the optical axis direction. Thus, the objective mount is guided so as to be movable with respect to the base in the optical axis direction. The objective mount and the base are held by four guide springs in a closely-attached state. Further, a focus screw rotatably held at a fixed position of the base is screwed into a female screw formed in the objective mount, and a pair of objective lenses moves in the optical axis direction by rotating the focus screw.
However, since the pair of binoculars disclosed in Japanese Patent Laid-Open No. 10-319325 has a configuration where a support mechanism and a guide mechanism of the objective mount entirely slide, the drive load is easily higher.
In order to increase a moving amount of the objective lens, it is necessary to lengthen an optical axis direction length of each of the two guide holes for guiding the objective mount in the optical axis direction. However, because a space around the objective mount and the base in the optical axis direction is limited, the gap between the two guide members provided on the base in the optical axis direction has to be narrowed. Since the guide member engages with the guide hole so as to be relatively movable in the optical axis direction, there is a backlash between them. Therefore, when the gap between the two guide members in the optical axis direction is narrowed, the position accuracy is deteriorated in moving the pair of objective lenses and the frictional force acting on a slide portion between the guide member and the guide hole also increases. Furthermore, because each of the focus screw and the female screw is integrally provided in the base and the objective mount, the frictional force on a slide portion between the objective mount and the base increases unless the position accuracy of both screwed portions is high.